This invention relates to an electrode with a long-time stability and a double-layer capacitor formed thereof.
A double-layer capacitor comprises selected electron conductors and ion conductors. The ability to store energy is caused by the capacitance of the interface between the electron conductors and the ion conductors.
In principle, two highly conductive electrodes, coated with an electroactive layer of a high capacitance, dip into a highly conductive electrolyte. When an outside voltage U is fed between the electrodes, the electrochemical capacitor is charged by the fed energy W which is half of the product of the capacitance and of the difference of squares of charge and discharge voltages U.sub.1, U.sub.2 : EQU W=1/2C(U.sub.1.sup.2 -U.sub.2.sup.2)=1/2CU.sub.1.sup.2 (1-.alpha.)=1/2QU.sub.1.sup.2 (1-.alpha..sup.2) EQU W=1/2C(U.sub.1.sup.2 -U.sub.2.sup.2)-1/2CU.sub.1.sup.2 (1-.alpha.)=1/2QU.sub.1.sup.2 (1-.alpha..sup.2)
wherein .alpha. is the fraction of the fed voltage which remains in the capacitor.
When the internal resistance of the capacitor R.sub.i against the external resistance of the consuming device R.sub.v is not negligible, this further reduces the usable energy by the ratio R.sub.v /(R.sub.v +R.sub.i).
The pseudocapacitance C of the electrode/electrolyte interface is composed of the proportion of the double-layer capacitance and the capacitive effects of electrochemical redox operations and adsorption processes.
FIG. 1 shows the basic structure and the electrotechnical equivalent circuit diagram of the double-layer capacitor. Each electrode is assigned to a parallel connection of a polarization resistance (R.sub.A or R.sub.K) and a capacitance (C.sub.A or C.sub.K). An ion conductor with the electrolyte resistance R.sub.e1 is situated between the electrodes. Together, the anode, the cathode and the electrolyte cause the measurable capacitance of the double-layer capacitor.
An electrochemical capacitor comprises at least two series-connected electrodes and thus at least two capacitive electrode/electrolyte interfaces. The cell capacitance is half as large as the capacitance of the individual electrode. However, the stored energy, since it is determined by the cell voltage, is twice that of the individual interfaces.
It has heretofore been known to use accumulators for storing electrical energy. However, the accumulators cannot be charged and discharged with any arbitrary frequency and, in addition, they can be only very slowly charged and discharged. In contrast, the double-layer capacitor can be cyclized with a frequency that is higher by orders of magnitude. In contrast to the commercially available electrolytic capacitor, the double-layer capacitor has a much higher specific capacitance and is therefore capable of storing higher amounts of energy per unit of volume and unit of mass. In a manner different than in the fixed-oxide dielectric of the electrolyte capacitor, in the case of the double-layer capacitor, electric energy is stored in the electrochemical double layer on the fixed-electrode/liquid-electrolyte interface. Additional advantages of the double-layer capacitor are: high energy and power density, high volumetric efficiency, maintenance-free operation, harmlessness in the inoperative state.
Materials on a base of conductive polymers function only at low currents and exhibit problems with respect to long-time stability. Carbon systems exhibit high resistances and poor efficiency. Japanese developers, such as NEC, ASAHI and MATSUSHITA, are therefore pursuing milliampere and microampere applications, among others, as a battery replacement for computer memories and consumer electronics. Others use proton exchange membranes which are coated with a mixture of ruthenium oxide and carbon, for example, GINER INC., U.S.A. These double-layer capacitors are not suitable for high-power energy stores.
Double-layer capacitors for higher currents, which are developed in the U.S. for SDI-applications, are also based on carbon technology (MAXWELL).
An electrode described in U.S. Pat. No. 5,079,674 consists essentially of carbon and plastic. Carbon particles are mixed with metallic salts. By means of the addition of a lye, metal hydroxides are formed which adsorb on the carbon particles. By means of a fluoropolymer, such as PTFE, the particles are then bound to one another and are dried at 80.degree. to 125.degree. C.
These known metal hydroxides cannot be used in an acidic solution and chemically are not very stable and are therefore not practical for use in pulse-operated double-layer capacitors. Only a thermal after treatment above 300.degree. C. will create stable metal oxides which can be used in an acidic and alkaline solution.
The compound of carbon, polymers and metal salts results in electrodes of a relatively high resistance. By means of these high-resistance electrodes, a pulse storage application subjected to high currents is not possible.
There is therefore needed an electrode, and a double-layer capacitor formed thereof, which have an improved conductivity, capacitance, energy density and power density.
These needs are met by an electrode with a long-time stability, a high surface capacitance and a low resistance for a double-layer capacitor. The electrode includes a chemically stable support material; an active layer of understoichiometric hydrated metal oxides; a base layer of precious metals on the support; and an electrically conductive oxidic intermediate layer between the support and the active layer.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.